The increased recognition of the deleterious health effects of chemical and biological pollutants, which often are in the form of particles of less than one micron in diameter, has created an urgent requirement for filtration media capable of removing such particles in an efficient and economical manner. There is a wide range of filter media available for removal of particles from air used for breathing or other processes requiring specific air purity standards. However, most of these media lose efficiency rapidly or increase in price and investment and operating costs, as particle filtrate size decreases.
A major portion of such increased cost is due to costs associated with the increased resistance of the filter to air flow as the efficiency of particle removal is increased. This creates a higher pressure drop for movement of air through the filter thereby imposing a greater work requirement upon the filter fan. This, in turn, can require the use of larger and more costly fans which consume greater amounts of energy which can make the entire process economically prohibitive. Also, filters capable of removing sub-micron diameter particles generally tend to rapidly plug up, necessitating frequent and costly filter replacement.
The most common filter media for air filtration consists of fibrous webs of fine fibers laid flat so that most are perpendicular to the direction of flow of the air being filtered. In the absence of electrostatic charging effects, equations describing the laws of fluid dynamics for particulate solids in an air stream have been found to provide good predictions of filter performance. Such physical laws and associated equations have been described by L. B. Torobin in "Momentum Transfer in Solids-Gas Flow", Ph.D. Thesis, McGill University, 1960 ("Torobin Thesis"), incorporated herein by reference.
The effect of fiber size on air or gas filter collection efficiency has been calculated by W. C. Hinds, in "Aerosol Technology", Wiley, 1982, p 184, using the appropriate solids-gas fluid dynamics equations with parameters set for conditions typical in industrial, commercial, and domestic air filters. The calculations reveal that, if sub-micron diameter fibers could be incorporated into filtration media in a uniform manner, they would be very effective in removal of sub-micron diameter particles from air streams. For a typical pollutant particle of 0.3 micron diameters entrained in air flowing through a filter made up of 10 micron diameter fibers, approximately 30 percent of the pollution particles were not removed by the filter. When the fiber diameter was reduced to 2 micron diameters, which corresponds to the lower limit of most industrial filters available today, the amount of unfiltered particulate matter decreased to approximately 17 per cent. Hinds projects that, if the media could employ fibers with a diameter 0.5 micron diameter, the filter would trap all but 3 percent of the incoming pollutant particles, while requiring only a relatively low pressure drop to sustain the flow of the gas being purified through the filter.
A number of significant problems, however, have inhibited the use appreciable quantities of sub-micron fibers in filter media. For example, one problem has been the difficulty of achieving uniform deposition of sub-micron diameter polymeric fibers onto a collecting surface. This is due, in part, to their tendency to to fly about in the fiber formation process. Because of their small size and low density, the sub-micron diameter polymeric fibers are extremely light and are therefore readily entrained in the stray large turbulence scale air currents, which are induced by essentially all fiber formation and collection processes.
This is in keeping with the theoretical predictions which show that currents with only one tenth to one hundredth the velocities needed to entrain the fibers above one micron in size are enough to carry off the sub-micron fibers. (Torobin Thesis at 228.) The dimensions of these currents or eddies and their velocities increase as the production process size is increased from the laboratory scale to commercial scale, and as the production processes are adjusted to make finer fibers. Accordingly, by conventional filter production methods, many of the sub-micron diameter fibers are not incorporated into the fiber web. Further, the stray fibers can damage or impair the filter production equipment.
Another problem inhibiting the use of appreciable quantities of sub-micron diameter polymeric fibers in filter media results from the greater tendency of polymeric sub-micron diameter fibers to attach to each other in flight during formation, forming undesired loose clumps, "ropes" or other agglomerates. This is a consequence, in part, of the unusually high ratio of fiber surface area to fiber volume characteristic of sub-micron diameter fibers. When this is combined with the relatively low density of polymers, the fibers tend to adhere to each other on contact.
The agglomeration problem is exacerbated by the presence of twisting and recirculating vortices in the ambient air which tend to twist the agglomerates into ropelike structures. The agglomerates eventually settle onto the formed fiber web where they constitute a source of significant non-uniformity. Another problem inhibiting the use of appreciable quantities of sub-micron diameter polymeric fibers in filter media results from the tendency of sub-micron diameter fibers to exhibit poor overall web mechanical properties and lack of self-support. This follows from the low tensile strength and relatively short length of the sub-micron diameter fibers. Sub-micron diameter fibers tend to only poorly attach to each other and are thereby susceptible to being lifted off and blown away by air currents associated with fiber production. Also, when such fiber webs are used as air filters, significant quantities of the sub-micron fibers are carried away intermittently by the air being purified. This results in unacceptable contamination of the air stream flowing through the filter.
Yet another problem retarding the use of sub-micron diameter polymeric fiber filtration media is the difficulty of uniformly distributing the ultra-fine fibers in the fiber web. If the sub-micron fibers are not uniformly distributed, much of the air being treated by the filter will not be subjected to the high efficiencies imparted by the sub-micron diameter fibers. Moreover, because the higher surface area associated with each region of high concentrations of the sub-micron diameter fibers imparts to it a relatively high resistance to air flow compared to regions of low concentrations, the ultra-fine fibers in these regions tend to be bypassed by the air being filtered, thereby reducing their effectiveness.
In addition to non-uniform distribution over the length and width of the fiber web, there is also a tendency for the sub-micron diameter fibers to collect at a certain depth of the fiber web. This results in premature plugging of the filter, since most of the solids separated by the filter collect in a thin plane.
It was found that the aforementioned difficulties encountered with sub-micron diameter polymeric fibers could be avoided to some degree when the fibers were made from higher tensile strength and higher density materials such as glass. Consequently, many of the air filters used today include sub-micron glass fibers. Sub-micron diameter glass fibers have been produced and incorporated in filtration media, as described, for example, in U.S. Pat. No. 4,548,632.
However, experiences with asbestos fibers has led to concerns about the possibility of adverse health effects resulting from the inhalation of such fine glass fibers. Also, pollutant-laden, used glass fiber filters cannot be disposed of by incineration. They constitute a hazardous waste which must be handled, at considerable expense and risk, in land fills. In response to these concerns, filtration media made from polymeric materials composed of fibers larger in diameter than their glass counterparts are displacing glass media, notwithstanding the poorer performance characteristics of the larger diameter polymer fibers. Consequently, a need exists in the air filtration industry for a filtration media exhibiting the benefits of sub-micron diameter fibers, without the aforementioned problems associated with the production of sub-micron polymer fibers or the health concerns associated with glass fibers.
The production of fine fibers from polymeric melts has been the subject of many efforts during the last half century. Research in the early 1950's by Van A. Wente at the Naval Research Laboratory led to a major fiber making procedure referred to as "melt-blowing." It is summarized in NRL report 5265, 1959. Conventional melt-blowing processes employ the fiber-forming polymers in molten form i.e. as a "melt" extruded through extremely small diameter orifices in a heated die. The resulting liquid jets of molten polymer are introduced into high-velocity blasts of heated air external to the liquid jets. The high velocity gas blasts stretch and attenuate the molten polymer jets into fine discontinuous "blown" fibers which are then collected on a moving surface. Although the early work resulted in finer fibers than had been made previously, they were still an order of magnitude above the sub-micron diameter diameters needed for efficient air filtration of sub-micron diameter particles.
Others improved the melt-blowing process making it possible to manufacture fibers generally in the 2 to 10 micron diameter range with a small quantity of sub-micron diameter fibers. Examples of patents describing fibers produced by melt-blowing processes include Hall (U.S. Pat. No. 2,374,540), Manning (U.S. Pat. Nos. 2,411,659; 2,411,660; and 2,437,263), Marshall (U.S. Pat. No. 2,508,462), Francis (U.S. Pat. No. 2,464,301); Ladisch (U.S. Pat. Nos. 2,571,457 and 3,017,664); Hartmann et. al. (U.S. Pat. No. 3,379,811), Hartmann (U.S. Pat. No. 3,502,763), and Buntin et. al. (U.S. Pat. Nos. 3,595,245, 3,615,995, 3,849,241, and 3,978,185); and Buntin, (U.S. Pat. Nos. 3,755,527, and 3,972,759).
The most common method of providing the external heated blast air for melt-blowing fiber production is accomplished through the use of two dimensional slot jets of various geometries and at various locations. Alternatively, the external heated blast air is provided via one dimensional jets surrounding each of the liquid polymer jets, as described in, for example Balaz (U.S. Pat. No. 4,047,861), Tunica, et. al. (U.S. Pat. No. 4,548,632), and Schwarz (U.S. Pat. Nos. 4,380,570, and 5,476,616).
In U.S. Pat. No. 3,972,759, Buntin describes an improved melt-blowing process for making fibrous webs which contain an average fiber diameter of 2 micron diameters, with an undisclosed percentage of fibers as low as 0.5 micron diameters. However, Buntin further notes that very low melt viscosity and very high gas velocities are required to make even a small fraction of sub-micron diameter fibers. Buntin reports that the required gas blast caused "extreme" fiber breakage and caused many fibers to be blown away from the laydown zone so that the fibers produced could not be incorporated into a practical fibrous web. Further, the large amount of heated gas required in the process reported by Buntin tended to make melt-blowing uneconomical for sub-micron diameter fiber production. Similarly, Joseph et. al. (U.S. Pat. No. 5,207,970) report increased difficulty in obtaining webs having uniform properties as the blast air rates were increased. Compared with other applications of fibrous webs, those used for filtration must achieve an high level of uniformity in order to maintain filtration efficiency. In addition to uniformity, the melt-blowing processes do not have means to overcome the combined problems of web weakness, fiber agglomeration and tendency for detachment from the web inherently associated with sub-micron fibers.
The problem of poor mechanical strength of filter webs containing substantial quantities of weak fibers has been addressed by a number of methods. Generally, these methods have been successful only when applied to polymeric fibers greater than one micron in diameter, but they have been considered for sub-micron fibers. The most direct method practiced to support weak fiber webs has been to support such webs by means of one or more layers of sufficiently strong fabric made from a diversity of materials. For example, Sexton et. al.. (U.S. Pat. No. 3,710,948), describes the use of an inner and outer layer of self-supporting and porous "scrim" fabric to support and contain an inner layer of relatively weak non-self sustaining fibers, which constitute the filtration medium.
Scrim reinforcing fabrics are often made from monofilaments of resin bonded together by heated calenders. Carey (U.S. Pat. No. 4,011,067) reports collecting a thin layer of about 0.2 ounce per square yard of sub-micron fibers on a scrim material and then covering it with a second scrim layer. This is much thinner than filters for general use, which require filter fiber weights of about 0.5 to 4 ounce per square yard.
Brock et. al. (U.S. Pat. No. 4,041,203) describes the use of a scrim to support fibers less than 10 microns, and they avoid calendering by the use of intermittent bonds. Other examples of the use of layers of handleable self-supporting porous materials to support weak fibers or protect them from abrasion are described by Prentice (U.S. Pat. No. 4,078,121), Kitson, et. al. (U.S. Pat. No. 4,196,245), and Bosses (U.S. Pat. No. 5,080,702). In combination with the use of supporting layers, methods have been reported in which the resulting system is impregnated with a bonding agent an example being Maddern, et. al. (U.S. Pat. No. 5,589,258).
The use of self-supporting porous layers of materials to overcome the inherent weakness of sub-micron fibers does not alleviated the problems of clumping, roping, segregation, and entrainment in stray air currents which occurs with sub-micron fibers prior to deposition on any surface. Even when the combination has macroscopic good strength, the weak inner core of sub-micron fibers has been found to develop cracks and regions of separation in the vibration that occurs in certain filter uses, due to local weakness of the fiber-to-fiber bonds. This allowed the air to be filtered to bypass or "short" through the filter, with a corresponding degradation of performance to an unacceptable level. In addition, calendering with a pressure sufficient to cause fiber-to-fiber fusion and the use of bonding agents can tend to result in excessive degradation of filtration performance when applied to sub-micron fibers. Because of their weakness, calendering causes them to compress into a plane with high resistance and low dust holding ability. Where a binder is used, the high surface area of the sub-micron fibers causes them to soak up too much binder and become partially clogged.
A second general method for addressing the poor strength of filter webs containing significant quantities of weak fibers has been to sequentially deposit and thereby sandwich layers of the weak fibers between layers of fibers whose diameters and lengths are sufficient to impart adequate strength to the composite matrix. Each layer is produced in a separate one-dimensional linear melt-blowing die with its associated gas blast generators. To make the layered media, the dies and their associated gas blast generators are generally positioned to be in a plane parallel to the plane of a moving foraminous belt and are set perpendicular to the direction of movement of the belt. The dies and their associated gas blast generators are typically placed in series with each other in a downstream direction. Some mixing can occur between the relatively weak fibers and the strong fibers. Examples of such arrangements are disclosed by R. R. Buntin, TAPPI, 56 (1973) 74; M. Ahmed (Polypropylene Fibers Science and Technology, 1982, Elsevier Co.,pp 437-440), Prentice (U.S. Pat. No. 4,078,124), Shipp et. al. (U.S. Pat. No. 4,714,647), Tani et. al. (U.S. Pat. No. 4,983,193) and Schwarz (U.S. Pat. No. 5,4766,616). Ahmed points out, however, that such arrangements lead to layers which are poorly attached to each other, unless the fiber bed thickness is considerably greater than the thickness required by filtration media. Poor attachment of the structure forming layers to the layer of weak fibers would mean that the weak layer would be unprotected with respect to fibrous web tensile forces.
The poor attachment of the layers as one reduces fiber diameter results, in part, from the inability to space the conventional melt-blowing dies close to one another in the down stream direction. In the standard melt-blowing system, a large amount of air is used in the "blast" jets. Accordingly, there must be sufficient distance between the dies so that the blast air of one does not disturb the layers already deposited or blow away the fine fibers contained therein, Also, there must be sufficient space to provide duct means to remove this air flow and the ducting can trap increasing amounts of fibers which may fall off as undesirable large agglomerates onto the fibrous web.
The results reported by Nissen et. al. (U.S. Pat. No. 5,260,003) for conventional and improved melt-blowing technology show that the blast air energy requirements increased exponentially from an average of 4 kilowatts per kilogram of fiber produced for fibers of about 3.2 micron diameter, to approximately 38 kilowatts per kilogram of fiber produced for fibers of 1.3 micron diameter. For the same fiber diameter reduction, it was necessary to reduce the overall fiber production rate for each die orifice from 0.12 pound per hour to 0.01 pound per hour. This suggests that adaption of melt-blowing geometries for the commercial production of sub-micron diameter fibers would be prohibitively expensive both in operating and equipment costs.
A third general method of forming webs which incorporate weak fibers is to cause their intermingling with reinforcing fibers. Page (U.S. Pat. No. 3,981,650) describes a modified melt-blowing die which allows the feeding of two different polymers to alternate orifices. This results in the production of intermixed compositions. It would be impractical to attempt to adapt this method to the production of intermixed fibers of sub-micron diameter fibers with fibers large enough in diameter to be reinforcing, since it would require the close spacing of blast jets with one or two orders of magnitude difference in velocity.
Anderson, et. al . (U.S. Pat. No. 4,100,324) suggests turbulently mixing melt-blown fibers with relatively large wood pulp fibers in air to form a matrix. In the process disclosed, the polymer fibers are required to be of sufficient strength to hold captive and interconnect the cellulose fibers and sub-micron polymer fibers would be too weak and too short for this purpose. Also, the high blast velocities required to form sub-micron fibers tend to blow the bulk of the relatively large cellulose fibers away.
General methods have been proposed for mixing weak and strong fibers after their formation and bonding the two together with binders. Binders in the form of particulates tend to plug filtration surfaces, however. This problem can be addressed, to some extent, by the incorporation of reinforcing fibers which have a lower melting point than the weak fiber constituents of a mixture of fibers. Bonding is achieved by heating the reinforcing fibers above their melting point and applying pressure to the web. An example of this method is given by Jackson, et. al. (U.S. Pat. No. 5,480,466). It would be difficult to adapt this procedure to sub-micron fibers since they would be in a clumped state upon collection prior to mixing, and the required uniformity of dispersion for an efficient filtration web would be difficult to establish.
Bean, et. al. (U.S. Pat. No. 4,268,235) describes a general process for mixing fibers of various sizes to form a web and then insuring uniformity of distribution by the use of parallel banks of oscillating brushes. While this method may be applicable to relatively large fibers, sub-micron fibers clump together when placed in contact with each other and it would be impractical to attempt to separate them after their collection and attempt to distribute them in a web containing larger fibers. Also the gross inter-mixing of sub-micron fibers with the larger fibers can cause discontinuities in the bonds between the larger fibers so that the resulting web would be too weak for use as a practical filter medium.
An alternative approach has been disclosed to produce sub-micron diameter fibers in which the polymer used to produce the fibers is first dissolved in a solvent and the solution is fiberized by conventional means or allowing the solvent to boil due to a sudden release of pressure. Although the resulting solution fibers are above one micron in diameter, the fiber diameters shrink as the solvent is evaporated or boiled. The resulting fibers may be sub-micron in diameter and they may be immobilized by bonding to each other due to the solvent action. Examples are described by Pleska, et. al. (U.S. Pat. No. 4,081,226), Raganato et. al. (U.S. Pat. No. 4,025,593, and, 4,189,455), Di Drusco, et. al., (U.S. Pat. No. 4,211,737), Shin (U.S. Pat. No. 5,032,326), and Nishioi, et. al. (U.S. Pat. No. 5,290,626).
Unfortunately solvent based processes are prohibitively expensive for general use. In addition, the solvents can present the potential for pollution if they escape the enclosures of the process.
Electrostatic charging of fibers used in filtration has been used in attempts to overcome the relatively low efficiencies of the existing polymeric media made from relatively large diameter fibers. The media thus produced are referred to as "electrets" and their use is reviewed by J. Van Turnhout in "Electret Filters for High Efficiency Air Cleaning" Journal of Electrostatics, 8 (1980). A recent example of such practice is disclosed by Tsai, et. al. U.S. Pat No. 5,401,446. Unfortunately electrostatic charge may leak away during storage or use of polymeric fiber media made from relatively large diameter fibers. Also, the electrostatic charge in such media tends to concentrate on its outer surfaces. The solids stopped by the charged filter tend to collect in a surface plane. Consequently, the interior volume of the filter may not be used efficiently, resulting in a relatively low dust holding capability. M. D. Bosses, however, in "Evolution of Improved Vacuum Cleaner Bags" Second Annual TANDEC Conference, 1992, reports that sub-micron diameter fibers "dramatically enhance " the capacity of electrostatically charged melt-blown media, adding further incentives for the development of media which contain uniformly distributed and properly anchored sub-micron diameter fibers.
In recent years, alternative processes have become available which have the ability to produce discontinuous sub-micron diameter fibers from molten materials while avoiding some of the problems associated with the conventional melt-blown process, particularly the problems associated with the large volumes and velocities of heated blast air and the low production rates of each orifice. Examples of the alternative processes are disclosed by Torobin in U.S. Pat. Nos. 4,363,646; 4,415,512; and 4,536,361. None of these examples disclose practical methods for preparing commercial-sized sheets of composite fibrous webs of polymeric materials which contain significant quantities of uniformly distributed and attached sub-micron diameter fibers. A need in the industry exists for a method of and apparatus for producing such composite fibrous webs which cost-effectively avoid or resolve the aforementioned problems.